It is well established that the insertion of arterial and venous catheters for various purposes such as for angiography can be responsible for patient discomfort. Locating and penetrating arteries and veins can be especially difficult when dealing with patients who are obese or present unusual anatomy.
Arterial and venous catheters are particularly useful for cardiac catheterization and other radiologic procedures such as cerebral angiograms.
The potential utility of Doppler ultrasound for accurately guiding the needle into a vessel has been recognized. Most applications utilize the transmission of ultrasonic waves through the needle and reception of ultrasonic echoes by a separate transducer located on the body of the patient separated from the syringe and needle. Such applications obviously have limited accuracy. For example, U.S. Pat. No. 3,556,079 directed to a "Method of Puncturing a Medical Instrument Under Guidance of Ultrasound" discloses in one embodiment the placement of both the transmitting and receiving transducers in the needle and syringe. Such an embodiment, however, requires a special catheter construction and can give an erroneous signal when the needle engages the blood vessel before penetrating the vessel.
A major advance was made to this technology by virtue of U.S. Pat. No. 4,887,606 directed to "Apparatus For Use in Cannulation of Blood Vessels." This patent teaches the use of a transducer insert positioned within a hollow needle including an ultrasonic transducer at one end for transmitting and receiving ultrasonic waves through the sharpened end of the needle. Upon location and penetration of a blood vessel, the transducer insert is removable from the needle for implementation of the known Seldinger technique for placing a catheter in a blood vessel.
FIG. 1 depicts, in cross section, a device which is the subject of U.S. Pat. No. 4,887,606. In referring to FIG. 1, needle 10 is shown as having sharpened end 11 and, located therein, ultrasonic flow sensing assembly 12. The assembly 12 includes a plastic support member 13 through which a first conductor 14 extends into contact with an electrode 15 on the back surface of transducer 16. Transducer 16 is affixed to support member 13 by means of a low impedance epoxy 17 which is filled with glass microballoons (not shown). A second conductor 18 is formed on the exterior surface of support rod 40 by means of metal deposition and extends into contact with electrode 15 on the front surface of transducer 16. The conductors 14 and 18 form a coaxial cable and the outer shield conductor 18 can be grounded during use.
An insulative material 19 such as an epoxy is formed around the periphery of the transducer 16 to electrically isolate electrode 14 from conductor 18 which is in turn connected to electrode 15. Transducer 16 is positioned near the distal sharpened end 11 of needle 10 for the transmission and reception of energy through the opening in the distal sharpened end of the needle.
Although the device disclosed in U.S. Pat. No. 4,887,606, the disclosure of which is incorporated by reference herein, represents a superior apparatus for cannulation of blood vessels, such apparatus is difficult to manufacture and at times provides a device whose sensitivity is somewhat low and, ideally, could be improved upon.
The present invention provides an apparatus for the cannulation of blood vessels which is not only easier to manufacture but is also of higher sensitivity than the device shown in U.S. Pat. No. 4,887,606.
These and other advantages of the invention will become more apparent when considering the following description of the invention and the accompanying exemplary drawings.